Help with Time Machine that has just stopped working due to encryption issues.

Time Machine suddenly stopped backing up due to an encryption error — iMac 24-inch, macOS 27.0

That sounds really frustrating, especially when everything was working perfectly yesterday. The good news is that this error does not necessarily mean your backups are lost or that the drive has suddenly become unencrypted. 

We first need to establish whether the problem lies with macOS, the backup drive, its encryption settings, or the connection.

The most important thing is not to erase, reformat or initialize the backup disk. Your existing backups may still be intact, so let us avoid doing anything that could put them at risk.

Start with the exact error message

If possible, write down the full error message or take a screenshot. There Is an important difference between Time Machine reporting that encryption is disabled and saying that an existing backup cannot be encrypted, unlocked or accessed. 

Each points towards a different possible cause.

If Time Machine settings will not open or respond

Try restarting your iMac normally, then open Apple menu >> System Settings >> General >> Time Machine again.

If System Settings itself has frozen, press Option–Command–Esc, select System Settings and choose Force Quit. Reopen it and try again.

If the settings still will not open, do not keep attempting to change the encryption settings. Make a note of the problem and include it when contacting Apple Support.

Check the backup drive or network connection

If you are ‘using an external USB or Thunderbolt drive, check that its power and cable connections are secure and connect it directly to the iMac rather than through a hub, if possible.

If the drive is visible in Finder and Time Machine is not backing up, you can try ejecting it through Finder. If it ejects successfully, disconnect it, restart the Mac and reconnect it once macOS has loaded. If it will not eject, is not visible or appears to be active, do not force it or unplug it.

If you are backing up to a NAS or network drive, check that the device is switched on, the network connection is working and the backup destination is accessible. Do not physically disconnect a network device as part of this step.

Check Disk Utility

Open Applications >> Utilities >> Disk Utility, then choose View >> Show All Devices.

Check whether the backup disk appears and whether any errors are reported. You can inspect its status without erasing it. If the disk appears damaged or is repeatedly disconnecting, stop before attempting repairs that could affect your existing backups.

Check the encryption password and disk format

If macOS asks for the backup password, try the original password. You can also open Keychain Access and search for the backup disk's name to see whether a saved password entry exists.

The disk's format may also be relevant. Apple supports several formats for Time Machine, including APFS and certain Mac OS Extended formats. Knowing which format the existing backup uses may help identify the cause, but the error alone doesn't establish that the format is the problem.

Avoid switching encryption off and on, removing and re-adding the backup destination, or starting a new backup setup until the existing backup has been assessed. Depending on the destination and the operation, you could lose the previous backups.

Check for macOS updates

Your post specifies macOS 27.0 on an iMac 24-inch. Check System Settings >> General >> Software Update for any available updates, particularly if the problem began after a restart or software update.

This does not mean macOS is necessarily responsible, but the timing may help Apple Support diagnose the issue.

Apple's official troubleshooting guidance is available here:

• If you can't back up or restore your Mac using Time Machine

• Time Machine troubleshooting on Mac

• Keep your Time Machine backup disk secure

If the problem persists and you cannot access Time Machine settings, I would contact Apple Support before attempting any more drastic remedies.

Usually this happens when an OS update like Golden Gate turns on FileVault when you weren't previously using it. Due to the design of modern computers, low-level disk encryption is always enabled. But the system stores the plain text password in firmware to effectively break the encryption.

But for some reason or another, that plain text password can get erased during the upgrade and now your system thinks you have FileVault enabled. I guess you do now. But since your backup isn't also encrypted, it's complaining about that.

Solutions are:

1) Turn off FileVault on your computer.

2) Buy a new backup hard drive and turn on encryption.

You could also turn on encryption with the old backup hard drive, but I don't recommend that. That could take a very, very long time. People hang on to back up drives for too long anyway. So I recommend just buying a new one.

First of all thanks very much for taking the time to look at my problem and send me some ideas for a solution.

I struggled through yesterday afternoon with all sorts of work throughs I could find on Safari, most were AI generated I think, but at the of the day I have a working Time Machine again. I got a message when it started working again that I was backing up from an encrypted disk to an unencrypted disk which has always been the case actually but it works.

I have just requested another backup now and it ran smoothly and completed properly so what more could I ask for.

I may be a bit of a novice at dealing with such problems but I sure learnt a lot yesterday.

Thanks again and have a nice day.

You may find this response very similar to 1 I just posted for some else who took the time to try and help me out and I appreciate your endeavours as well so thanks very much for taking the time to look at my problem and send me some ideas for a solution.

As I indicated previously I struggled through yesterday afternoon with all sorts of work throughs I could find on Safari, most were AI generated I think, but at the of the day I have a working Time Machine again. I got a message when it started working again that I was backing up from an encrypted disk to an unencrypted disk which has always been the case actually but it works.

I have just run another backup now and it progressed smoothly and completed properly so what more could I ask for. At the end of the day the only thing I could see that happened was my Time Machine got itself into a loop it could not get out of by itself and needed help.

Being a novice at dealing with such problems really spiked my learning curve but I did learn a lot yesterday.

Thanks again and have a nice day.